Providing and maintaining adequate concentrations of bioactive agents, such as drugs, in the pre-corneal tear film for extended periods of time is one of the major problems plaguing methods and systems for ocular drug delivery. The delivery of such bioactive agents from a contact lens has been examined. This dosage form generally suffers from a phenomena known as burst release, where most or all of the agent is released in a very short period of time after placing the contact lens in the eye, with only a negligible release over longer periods of time. An important contact lens material is a silicone hydrogel, which is a material that is employed for extended wear lenses that can be used for periods up to about a month. Hence, the development of methods and contact lens appliances, where the release of a drug occurs over an extended period of time, is attractive.
The release of a bioactive agent or any other compound from a contact lens is ultimately controlled by diffusion within the lens material. For a diffusion controlled process, the duration of release can be approximately calculated by l2/D, where l is the path length that a compound needs to traverse and D is the molecular diffusivity. For a typical contact lens, l is the thickness of the lens, which varies in the radial direction but is on average approximately 100 microns for a typical lens. The period of time over which a drug is released from a contact lens can be increased by either increasing l or by decreasing D. In most diffusion controlled systems, augmentation of diffusivity has been performed by changing the bulk material to one of a different diffusivity. However, because of the strict requirements of a contact lens where many material properties can not be compromised, there are practical limits to the selection of the bulk material. Furthermore, an effective strategy to modifying the diffusion process must be applicable to a wide range of bioactive agents with a similar bulk material.
One approach to the control of molecular diffusion in contact lenses has been disclosed by Qiu et al., U.S. Pat. No. 6,827,996 where a diffusion-controllable coating is placed on the surface of a lens. The method involves the deposition of one or more layers of a diffusion barrier on the surface of a lens. Deposition of a thin film on the surface can alter the surface properties, such as wettability, protein binding, and lubricity, which can have undesirable consequences. A single layer may not be sufficient to significantly alter the drug transport unless it is very thick, which can adversely impact transparency and mechanical properties. Construction of multiple layers complicates the fabrication process. Also, if the swelling of the deposition thin film differs from that of the bulk material, the shape of the coated contact lens can be altered from that of the uncoated lens making the process of lens design iterative and complicated. Although the need and statement of intent to control the release of drugs is disclosed, examples are provided only for preventing release of an agent and toward preparation of lenses that do not require an extraction process to remove monomers and other compounds involved in the formation of the lens.
An approach that attenuates drug release properties without impacting surface properties is desirable. Hence a method to modify the delivery of a bioactive agent from a lens remains a target in the development of a contact lens dosage form for bioactive agents.